1. Field of the Invention
The present invention relates generally to the field of fluid density measurements for a hydrocarbon or brine sample and relates in particular to an acoustic method and apparatus for determining the density of an unknown fluid such as drilling mud, brine, or a hydrocarbon in a downhole environment.
2. Summary of the Related Art
In various industrial processes that involve fluid material, it is useful to know the properties of the fluids involved. These fluid properties include, for example, density, compressibility, and acoustic impedance. Knowledge of the values of these various properties can be used to adjust process parameters or warn of impending calamity. In many applications, such as oil and gas well (borehole) drilling, fluid density is of particular interest. It is important to know or at least estimate the density of drilling fluid (also referred to as drilling mud) during a drilling operation, in order to prevent a blowout of the well caused by reduced mud weight resulting from incursion of natural gas into the drilling mud.
In a drilling operation, drilling fluid is pumped down the drill string (essentially a very long pipe) and exits at the drill bit. The drilling fluid then returns to the surface within an annulus formed between the outside of the drill string and the inside of the borehole. As the drill bit burrows into the geologic formations, the bit passes through zones containing various fluids. These zones can include saltwater, oil (hydrocarbons), lightweight fluids such as natural gas. If the pressure within the zone is greater than the pressure within the borehole, these fluids will enter the borehole and mix with the drilling fluid. As lightweight fluids mix with drilling fluid, the average density of the drilling fluid decreases. If the total weight of fluid within the borehole decreases too much, it can lead to a blowout when a high-pressure zone is entered. It is therefore very important that the density of the drilling fluid be accurately monitored. In producing wells, the local fluid density, along with other measurements, can be used to infer the proportions of oil, water and natural gas that the well is producing at various depths in the well.
Logging tools for measuring fluid density with varying degrees of success are well known in the art. One commonly known technique for measuring drilling fluid density involves the use of acoustic transducers, particularly ultrasonic transducers, as described in U.S. Pat. No. 4,571,693 (the '693 patent). The device described in the '693 patent provides an ultrasonic transducer coupled to the body of a probe. The ultrasonic device transmits and receives a first signal across a first solid/fluid interface and a second fluid/solid interface, in order to measure the sound velocity of the fluid. A second ultrasonic signal is provided as a reference signal generated by reflection off the surface of a void within the device that is hermetically sealed from contact with the fluid. Measurements of the first and second signals, as reflected off the two surfaces are used to calculate reflectance and acoustic impedance, from which density of the fluid may be inferred.
Other known methods and apparatuses for measuring the fluid density in boreholes, such as those described in U.S. Pat. Nos. 4,939,362 and 5,204,529, include the use of either chemical radioactive sources or electrically-controlled radioactive sources (e.g, pulsed neutron source), which present clear environmental and health hazards. It is therefore apparent that a need exists for an improved acoustic well logging tool and method to determine the density of drilling fluid and hydrocarbon sample associated with downhole operations in the borehole.